超臨界流體逐漸從學術研究走向實際應用，水的超臨界流體更是應用於核反應爐的製造，然而高溫高壓的環境中，對物質的影響仍屬於大量未知的灰色地帶，在傳統認知中，超臨界流體是勻相的，然而有些文獻表示，水在超臨界流體狀態下是非勻相的，本文透過理論計算研究在超臨界狀態下，水分子的空間分布得到論證；文獻中提及在超臨界水的狀態下，水的紅外線振動光譜會有明顯的超頻吸收，作為超臨界流體狀態的指標。
本文從少量水分子在固定能量的情況下，利用分子動力學模擬水分子於真空中的振動光譜模型，隨著水分子數的增加，水分子間的氫鍵作用逐漸明顯，同時觀察到在超頻區的吸收逐漸變弱，藉此說明在超臨界狀態下，水的紅外線吸收光譜在高頻區的明顯吸收，主要是有小型分子簇或單分子水的存在。其後以大量的水分子進行真實模擬，並在系統中搜尋小型分子簇的存在，研究超臨界狀態中是否存在大量的小型分子簇，以證實文獻中超臨界狀態下，水的超頻吸收。

It is noticed that supercritical fluid can play a special role in chemical reaction as solvent. Supercritical water becomes important role in manufacturing 4th generation nuclear power reactor. However, we still lack the knowledge of supercritical water due to the high temperature and high pressure. For example, it was consensus that supercritical fluid is homogeneous. Some works imply that supercritical water is heterogeneous. In this article, we reveal some property of supercritical water through theoretical calculation. With molecular dynamics, we would like to analyze the special distribution and the infrared-radiation (IR) spectrum for supercritical water.
Beginning with small amount of water molecule, we simulate water in vacuum. With the increase of the number of water molecule, it indicates that hydrogen bonding has strong effect on high frequency intensity which is overtone in IR spectrum. The overtone is relatively significant in supercritical water. According to the simulation with more number of water, we analyze the special distribution of water molecule in supercritical water. There is a large amount of small water cluster. It implies that the obvious overtone signal is caused by the small water cluster in supercritical water.

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